PEM fuel cells require humidification of both the oxidant reactant gas and the fuel reactant gas to prevent the membrane from drying out, thereby to avoid degradation of the membrane and of the fuel cell performance. One approach to humidifying the reactants is the use of water transport plates, which are continuously supplied with water, and which have water flow channels on opposite sides of the plates from the respective reactant gas flow field channels, thereby to humidify the reactants internally of the fuel cell. However, porous plates filled with water provide significant difficulty when they are routinely subjected to subfreezing conditions, because of the need to drain components to prevent mechanical damage, and because of the time and energy required to melt ice during startup procedures. This renders water transport plates less attractive for implementing fuel cells which are to be used for powering vehicles.
Fuel cells are designed and operated under conditions which result in the fuel and oxidant reactants exiting the cells saturated with water vapor relative to the related exit temperature. There are many known configurations for humidifying reactant gases prior to entrance of the gases into the fuel cell. One known configuration is illustrated in FIG. 1 in a fuel cell in which all of the fluids have multi-pass flow fields. Specifically, the fuel cell 11 has an internal coolant inlet manifold 12, and an internal coolant exit manifold 13. The coolant therein flows from the inlet manifold 12 to the right, and then flows through the center of the fuel cell toward the left, whereupon it flows rightwardly toward the fuel exit manifold 13, in generally-S-shaped channels, the direction of flow being demarcated by the dotted lines in FIG. 1. The fuel cell has an inlet manifold 16 for fuel reactant gas, which may be hydrogen or a hydrogen-rich stream obtained by reforming a hydrocarbon; the fuel flows rightwardly to a fuel turnaround manifold 17, after which it flows leftwardly to a fuel exit manifold 18; the rightward and leftward flow channels being demarcated by a dash dot line in the center of the fuel cell. The fuel cell 11 also has an air inlet manifold 21, for oxidant reactant gas, which may be oxygen but is more typically air, an air turnaround manifold 22, and an air exit manifold 23. The air flows from the inlet manifold upwardly to the turnaround manifold, and then flows downwardly to the air exit manifold 23, the upward and downward flow passes being demarcated by a solid line in the fuel cell 11. Inscribed on the fuel cell 11 are approximate local temperatures and the approximate local relative humidities.
In FIG. 1, conventional fuel recycling is achieved with a fuel recycle pump 26 which forces a fraction of the fuel exhaust from the fuel exit manifold 18 along a fuel inlet conduit 27 which also receives fuel from a source, such as a source of hydrogen 28. The amount of recycle is generally adjusted so as to achieve close to 100% fuel utilization, by means of a valve (not shown) which controls the amount of fuel that passes to an exhaust 29, which may be ambient or further fuel processing components. The degree of humidification of inlet fuel is limited to about 28% relative humidity relative to the local cell temperature because the cell temperature of the fuel inlet is higher than the dew point temperature of the fuel recycle stream.
Similarly, air is recycled in a conventional fashion by means of an air recycle pump, such as a blower 31, which returns a substantial portion of air from the air exit manifold 23 through an air inlet conduit 32 to the air inlet manifold 21. Fresh air is supplied from a source of air 33, which may be ambient and which is advanced through the conduit 32 to the air inlet 21 by means of an air inlet pump, such as a blower 34. The amount of recycle air is controlled by selection of the air recycle blower 31 and the inlet air blower 34 as well as by adjustment of an air exit valve (not shown) which controls the amount of air allowed to pass to exhaust 35, which typically is ambient. Typically, the maximum humidification that is achievable at the air inlet 21 is about 42% relative to the local cell temperature because the cell inlet temperature at the air inlet is higher than the dew point temperature of the air recycle stream; further increase in recycle air would cause too much of the inlet air to be oxygen depleted air, which would starve the cathode and deteriorate the power generation process.
A known variant for humidification of the inlet air utilizes an enthalpy recovery device 38 as illustrated in FIG. 2. In the enthalpy recovery device 38, the exhaust air from the air exit manifold 23 passes through flow field 39 to exhaust 35, on opposite sides of porous separator plates 40 from inlet air passing through flow field 41 to the air inlet pump 34. Temperature and partial pressure equalizations cause heat and moisture to be transferred from air in the flow field 39 across the porous separators 40 to air in the flow field 41. Although no oxygen-depleted air is utilized in this configuration, because the dew point temperature of the air exiting the manifold is below that of the air entering the inlet manifold, the inlet air cannot be humidified above approximately 42% relative humidity.
It has been found that 42% (for instance) relative humidity at the air inlet is insufficient to operate the fuel cell without providing liquid water to internally humidify reactants within the cell. Furthermore, membrane life is reduced at 42% compared with life at 100% relative humidity relative to the cell temperature at the air inlet.